GMC Repair: Transmission, throttle position sensor, new jasper

Question
Down shifting from 4th to 3rd....

I have replaced
computer, transmission, wiring harness

the problem ...  shifts out great for 15 to 25 miles then down shifts  ...  will not go back into 4th..   manuel shifting will not release
the shifting...   can stop and cut of engine from
a few minutes...  start driving and the shifting is great for a very short distance...
I'm at wits end ... what do you suggest..
PS the local dealer can not find any reading of cause from his computer... the dealer replaced my on board computer and now it runs worse than it did....  the mile on the unit 210,000 with
new Jasper moter and a Jasper trans....
2000  3500 cargo van ext

Answer
Hi,Lance

There is a GM bulliten that was issued for this complaint,it usually happened after a trans repair or replacement a few things can cause this problem but ill outline what the GM document says,it is up to you whether to attempt this yourself or have the dealer do it for you,ill give you the Doc number so you can present it to the dealership so they can refer to it,the GM doc number is 1351952.
What may be happening is that the throttle position sensor was not relearned and causing the downshift and delayed shift to 4th,i can give you the procedure but let me know what you are going to do first,whether returning to dealer to have this done or attempting it yourself.let me know,Thanks


ED